There are many kinds of rail road cars for carrying particulate material, be it sand or gravel aggregate, plastic pellets, grains, ores, potash, coal or other granular materials. These materials are not liquid, yet may in some ways tend to flow in a quasi liquid-like manner under the influence of gravity. Many of those cars have an upper opening, or accessway of some kind, by which the particulate is loaded, and a lower opening, or accessway, or gate, by which the particulate material exists the car under the influence of gravity. Others such as rotary dump gondola cars, may be unloaded by use of a rotary dumping system in which the entire railroad car is inverted such that the lading may be dumped out while the car is upside down.
In general, design of cars of this nature tends to involve a balancing of a need to reduce car weight to permit a greater mass of lading to be carried, and a need for relative simplicity of construction with a desire for robustness and long service life.